Motor vehicle transmissions generally include selectively engageable gear elements for providing multiple forward speed ratios through which the output torque of the engine is applied to the drive wheels of the vehicle. In automatic transmissions, the gear elements which provide the various speed ratios are selectively activated, as through fluid operated, friction torque transfer devices such as clutches and brakes. Thus, shifting from one speed ratio to another generally involves releasing (disengaging) the torque transfer device(s) associated with the current speed ratio and applying (engaging) the torque transfer device(s) associated with the desired speed ratio. Any torque transfer device to be released during a particular shift sequence is conventionally referred to as the off-going torque transfer device while the torque transfer device to be applied during that same shift sequence is referred to as the on-coming torque transfer device. There is generally a slight overlap between the "release" and "apply" of the torque transfer devices involved in a shift sequence, and high quality shifts are achieved only when the "release" and "apply" operations are properly timed and executed.
Conventionally, the shifting control effected by an automatic transmission is performed in conjunction with a logic control map and various inputs which reflect such system parameters as vehicle speed, engine throttle position and engine torque. Fluid pressure signals representative of the various system parameters are processed in an on-board computer and/or microprocessor to determine when a shift is in order and to actuate, in accordance with the logic control map, electronically controlled valves in the hydraulic control system which respond to the signals received from the computer to effect the required engagement and/or disengagement (and in the proper order) of the appropriate torque transfer devices necessary to secure the desired speed ratio changes to the output shaft of the transmission.
Conventionally, a pulse width pressure modulating valve may be employed in conjunction with each torque transfer device required to select the various speed ratios provided by an automatic transmission in order to eliminate the various problems associated with the overlap between the engagement and disengagement of the appropriate torque transfer devices. An exemplary installation of such an arrangement is disclosed in U.S. Pat. No. 4,790,513 issued to Davis et al. Dec. 13, 1988, and assigned to the assignee of the present invention. Installations in which a single pulse width modulating value is employed in conjunction with plural torque transfer devices are also known; see for example, U.S. Pat. No. 4,603,603 issued to Salmon Aug. 5, 1985, and assigned to the assignee of the present invention.